You must have the No One Left Behind challenge selected before you begin in order for this to count.
You start off with two revives, and you can earn another one by looting a large artifact from the sarcophagus in a crypt. You can tell you have 3 because all 3 crosses with circles in your top left will be filled in. Once you have enough, just let one person die (either by letting an enemy kill you or by killing yourself in some way (standing in your own fire arrows works well). Once one person is down, they will have a 59 second timer, and their partner can revive them by holding
over their body. Do this 3 times. It doesn't matter if the same person dies 3 times or if you split the 3 revives across both partners. Each time you use a revive, it should come up with a counter toward your challenge progress. Once you have done all 3, the achievement is yours!